The laser welding process has been developed as one out of many ways of joining metal parts. In welding processes in general, heat, pressure, or both it applied to melt a part of the metal and thereby to join the parts when the molten metal solidifies. The process can be carried out with or without filler material to produce a localized union through fusion or re-crystallization across the interface. The quality of a weld seam is largely governed by the weld joint penetration in a weld gap and by the homogeneity of the interface, e.g. the number of pores. The weld joint penetration depends on the temperature in the melt which again depends on the volume of the melt and thereby on the size of the gap between the parts which are joined. The pores derive from gases which are confined during the crystallization, and these pores, depending upon their number, size and location in the interface, can lead to cracks or prevent good adherence of a surface coating.
High energy density welding methods such as electron beam welding and laser welding have been used to achieve high depth to width ratios. In the laser welding process, the heat is generated by a concentrated coherent light beam which is focused on a very small spot. Often, the reflectivity is high. One problem related to the reflectivity is that the reflected beam may damage the optics or other parts of the laser welding equipment. Another problem is that the reflection prevents sufficient deposition of energy to melt the metal. In one attempt to solve the problem, e.g. as suggested in U.S. Pat. No. 5,760,365, the welding joint is prepared with a gap of one to several millimeters, and the welding variables are adjusted by use of a model of the laser beam and the geometry of the joint. The light beam reflects back and forth between walls of the gap which results in an increased localised heating of the metal. One problem, however, is that the gap increases the amount of metal which must be melted and thus decreases the speed at which laser can be advanced in the gap. If the laser beam energy is increased to solve this problem, a result will typically be an excessive heat input. If the welding gap is narrowed, the speed may be increased, but in that case, the laser beam must be positioned precisely over the gap, and if the gap is too narrow, the above mentioned problem with reflection of the laser light recurs.
As aforementioned, the size of the gap is important to obtain a reliable welding quality. The size of the gap is governed by the position of one part relative to the other, and in practice, it is often difficult to locate the parts precisely.